Few dinosaurs have the same mystique as Tyrannosaurus Rex, but the ‘tyrant king lizard’ that once roamed North America may have been misunderstood all this time.
A new analysis of the bones and teeth of 37 T-Rex specimens suggests the dinosaur may need to be reclassified into three different species—with the fearsome predator that lived 90 million to 66 million years ago potentially receiving two species of siblings: the lizard tyrant queen and tyrant emperor lizard.
A study, published in the journal Evolutionary Biology, says it has long been recognized that the “robustness” of adult tyrannosaur skeletons varies considerably. This was explained by sex differences—with female T-Rex perhaps smaller than their male counterparts.
Alternatively, this could be explained by stages of development, or simply individual variation in traits.
Other differences include banana-sized teeth — some T-Rex jaws have a single ‘D’-shaped incisor that is substantially smaller than the next tooth, while others have two of these smaller D-shaped teeth.
Scientists compared the length and circumference of the femur, or thigh bone, of 24 T-Rex specimens. They also measured the base of the teeth or the gap in the jaw to understand whether 12 of the dinosaurs had one or two fine incisor teeth. The team’s conclusion was that T. rex was not a single, unchanging dinosaur, but may have had two equally terrifying sister species.
“All three species were between six and seven tons with similar skulls and bodies. It would be like the difference between being attacked by a lion or a tiger,” said Gregory Paul, author of the study and the book “The Princeton Field Guide to Dinosaurs.”
The differences were “subtle,” similar to how “lion (Panthera leo) and tiger (Panthera tigris) skeletons are difficult to distinguish, even among experts, he said.
Variation of fossils vs different species
However, other paleontologists disagreed with the results.
“I understand the temptation to split T.-Rex into different species, because there is some variation in the fossil bones that we have. But ultimately, to me, this variation is very small and does not indicate a significant biological separation of distinct species that can be defined based on clear, explicit, and consistent differences,” said Steve Brusatte, Professor and Personal Chair of Paleontology and Evolution of the School of Geosciences at the University of Edinburgh, via email.
“Splitting the T-Rex into three species based on measurements of 38 bones is not a strong enough argument for me.”
Thomas Carr, associate professor of biology and director of the Carthage Institute of Paleontology at Carthage College in Kenosha, Wisconsin, said the characteristics identified by the researchers “represent nonsense variation, not biological signal.” A study he conducted on variation in T-Rex skeletons, published in 2020, did not reveal different species.
‘T-Rex for me’
The study authors said that the more robust tyrannosaurs in their sample outnumbered the “graceful” or slender tyrannosaurs by 2 to 1, and such a large disparity could not be explained by the small sample size, nor could it be a difference based on sex, the that would result in a more even split.
The variation in leg bones was also unrelated to developmental stages, they added, because sturdier femurs were found in some juvenile dinosaurs that were two-thirds the size of an adult, while slender leg bones were also found in some dinosaur specimens. adult size.
As far as tooth structure is concerned, specimens with an incisor tooth were correlated with thinner leg bones — although they only had leg bone measurements for 12 dinosaurs.
While the authors admitted that the data “does not meet the ideal proof” of three separate species, they said the study indicated that there were three recognizable “morphotypes” within the tyrannosaurs studied.
These were the now famous Tyrannosaurus rex (tyrant king lizard) with robust features and a smaller incisor; another stout dinosaur with two incisors, which the researchers called Tyrannosaurus imperator (tyrant emperor lizard), and a third, slender, tyrannosaurus they named Tyrannosaurus regina (tyrant queen lizard).
“The full expectation is that these new taxa will be tested and, if necessary, revised as additional samples and analyzes come to light,” the study said.
Placing animals that went extinct tens of millions of years ago into narrowly defined categories is inevitably challenging, but the authors said they hope the work will fit in and focus on future research. But Brusatte, from the University of Edinburgh, said he was not convinced.
“Fundamentally, this boils down to the age-old grouping versus division debate when classifying species. It is difficult to define a species, even for animals today, and these fossils lack genetic evidence that can test whether there were truly separate populations.
“Until I see much stronger evidence, these are still T-Rex to me, and that’s what I’m going to call them.”
Source: CNN Brasil
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